1. Field of the Invention
The present invention relates to a high-breakdown-voltage semiconductor device, particularly to high-breakdown-voltage semiconductor devices such as a power controlling static induction transistor, static induction thyristor, and diode.
2. Description of the Related Art
A semiconductor element for power control in which silicon carbide (hereinafter abbreviated as SiC) is used as a wide gap semiconductor material has superior characteristics of high speed and low loss as compared with a related-art element in which silicon is used. There has been a demand for practical use of the semiconductor element. In recent years, a large number of semiconductor elements using SiC, such as a Schottky barrier diode, MOSFET, and static induction transistor (hereinafter abbreviated as SIT) have been presented. It has been confirmed that the properties of the elements are far superior to those of an element using silicon.
A structure and manufacturing method of a related-art typical SIT will be described hereinafter. An n+ layer (source) is formed on one surface of an n− drift layer, a p+ layer (gate) is formed in the periphery, and further an n+ layer (drain) is formed on the other surface of the n− drift layer. Source, gate, and drain electrodes are disposed in the n+ layer (source), p+ layer (gate), and n+ layer (drain), respectively.
In the SIT, a current is passed between the source and drain, and is controlled by a bias of the gate. When a negative bias is applied to the gate, a depletion layer spreads. In this structure, a channel width through which a current passes is controlled to change a current value. That is, when the gate is positively biased with respect to the source, the spread of the depletion layer in the channel between the adjacent gates is small, the channel width broadens, and an on state is obtained. On the other hand, when the gate is negatively biased with respect to the source, the depletion layer spreads over the whole channel width between the adjacent gates, and an off state is obtained.
When the depletion layer is spread to control the current path, and a breakdown voltage in applying a reverse voltage is raised, it is necessary to deepen a gate region and narrow the channel width. On the other hand, in order to lower an on resistance per area of SIT, a ratio occupied by a source region with respect to an element region needs to be increased, and both the gate and source require fine processing. Moreover, it is very difficult to deepen the gate region because of a small diffusion coefficient of impurities in SiC.
To solve these problems, an element structure has also been proposed in which a trench is formed in a substrate and the gate is formed inside the trench (e.g., Henning, J. P.; Przadka, A.; Melloch, M. R.; Cooper, J. A., Jr.: IEEE Electron Device Letters, Volume: 21 Issue; 12 Dec. 2000, Page(s): 578–580). In this case, the following sophisticated processing technique is used to form the gate inside the trench. That is, the technique comprises: forming a metal film along a trench shape in the whole surface of the substrate including the inside of the trench; coating the surface with a resist; and allowing the resist to reflow and pool only in the trench. Thereafter., the technique comprises: using the resist pooled only in the trench as a mask to etch the metal film; and selectively leaving the metal film in contact with the bottom and lower side surface of the trench to form the gate electrode.
However, when the resist is allowed to reflow in the above-described method, there is a problem that the resist also easily remains outside the trench. In this case, when a pattern of the resist generated by the reflow is used as the mask to etch the surface, the metal film also remains outside the trench, and electric short-circuit or deterioration of flatness of the element surface is caused. There is a problem that manufacturing yield is deteriorated and element properties are also deteriorated.
On the other hand, for a switch-off property of a SIT, in the element in which the metal film is selectively left in contact with the bottom and lower side surface of the trench to form the gate electrode, the spread of the depletion layer in the width direction of the channel cannot be said to be necessarily sufficient. There is a problem that the switch-off property is not sufficient. This problem similarly exists in the switch-off property of a junction barrier Schottky diode (hereinafter abbreviated as JBS). In the JBS, a p-type semiconductor layer is selectively disposed around a Schottky junction, and the depletion layer is extended into an n-type high-resistance semiconductor layer from a pn junction between the p-type semiconductor layer and n-type high-resistance semiconductor layer so that the switch-off property is improved. In the element in which a control electrode is formed in the p-type semiconductor layer in the above-described method, it cannot be said that the spread of the depletion layer is sufficient, and there is a problem that the switch-off property is not sufficient.
Therefore, there has been a demand for realization of a high-breakdown-voltage semiconductor device in which superior element properties can be obtained and manufacturing yield is improved.